1. Field of the Invention
This invention relates to an improvement in and concerning a roller transporting printing web or sheets in a varying printing device (hereinafter it is called "printing web transporting roller"), and more particularly to an improvement in and concerning a roller such as a guide roller which is used in various types of rotary press.
2. Related Art
The printing technique consists in reproducing information composed of letters or other figures in a large quantity as hard copies portraying an identical image or printed matter on the surfaces of papers or other printing sheets or web. The printing devices which are used in the printing technique for forming the printed matter by depositing a coloring material such as ink on a printing plate and impressing the surface of a printing sheet or web to the plate and thereby transferring the coloring material onto the surface of the printing sheet or web, as widely known, vary according to the form of a printing plate and the mode of transfer of the ink from the printing plate to the printing sheet or web (direct printing or indirect printing system) among offset printing apparatus, letterpress printing apparatus, flexographic printing apparatus, gravure printing apparatus, screen printing apparatus, etc.
Though these printing presses vary according to the choice between requiring a printing plate to be directly impressed on a printing sheet or web and requiring an intermediate such as a rubber blanket to which the ink has been preparatory transferred from a printing plate to be impressed on a printing sheet or web, they share a major common concept of causing the ink on such a printing element (printing plate or intermediate) to be transferred to a printing sheet or web and they predominantly share in common the construction of a printing sheet or web impressing and transporting mechanism for pressing the printing sheet or web on the printing element and then transporting the printing sheet or web.
When a web, i.e., continuous strip, of paper wound up in the form of a roll is used for offset printing, an ink image is formed by causing an ink to be transferred from a plate cylinder to a blanket cylinder and then transferred by impression onto the surface of the web being passed between the blanket cylinder and an impression cylinder. The web having the ink image formed on the surface thereof is subsequently transported in changed directions by a plurality of guide rollers and passed along the interior of the press.
In the rotary press for continuously printing to the web of paper or film paid out of a roll, the web transporting system thereof is provided with numerous guide rollers. The transporting system using these guide rollers is furnished not only in the rotary offset presses (newspaper rotary offset press, commercial rotary offset press, and business form printing press) but equally in the web-fed rotary gravure rotary press, flexographic rotary press, rotary letterpress machine, etc.
Most guide rollers heretofore used in the rotary presses have been produced by forming iron pipes and plating their surfaces with chromium or faithfully repeating this procedure while using aluminum alloy pipes as bases in the place of iron pipes for the sake of decreasing weight. Further, the guide rollers which have their surfaces (1) knurled, (2) covered with a tape having a surface coarsened after the fashion of a sandpaper, or (3) coated with a ceramic layer deposited by thermal spraying for the purpose of preventing the printing sheet or web from slippage and protecting it against possible adhesion of ink due to the point contact effect have been heretofore known.
The guide roller which is an aluminum alloy pipe furnished with a knurled surface has such poor resistance to abrasion as suffers early loss of protuberances of knurl and consequent appearance of inclination to slippage. The guide roller which is covered with a tape having a surface coarsened after the fashion of a sandpaper is likewise incapable of offering a long service life because sand beads fall off the tape and the tape peels off after a short use. While the guide roller which is coated with a ceramic layer deposited by thermal spraying is highly resistant to abrasion and extremely effective in terms of resistance to slippage, it is at a disadvantage in developing the following problem of prominent inclination to adhesion of ink.
The surface of the guide roller heretofore known to the art is invariably formed of a material exhibiting a relatively high capacity for allowing the adhesion of ink. Even when the guide roller has its surface knurled, after a protracted operation of the rotary press using the guide roller, the ink image impressed on the surface of a printing sheet or web is transferred in the form of a transferred ink image to the surface of the guide roller held in contact with the printing sheet or web and the transferred ink image is reversibly transferred to the surface of the subsequent printing sheet or web and eventually suffered to smear the printed matter. In the operation of the rotary press which uses this guide roller, therefore, the guide roller must be periodically cleaned. The cleaning work interrupts the printing operation and imposes a heavy load thereon. If the cleaning work is neglected, the printing operation incurs the trouble of impairing the quality of the printed matter to be produced.
Particularly in the case of the guide roller which is coated with a ceramic layer formed by thermal spraying, the ink seeps into and deposits on the depressions which are formed on the coarse surface resulting from the thermal spraying. The deposited ink in the depressions is not easily removed by simply wiping the coarse surface. When the coarse surface is cleaned with a solvent, the ink dissolved in the solvent migrates by permeation into the pores in the ceramic layer formed by thermal spraying. Thus, the cleaning work is difficult to carry out.
Recently, for the guide roller under discussion, it has been proposed to use a roll which is made of fiber-reinforced plastics such as carbon fiber-reinforced plastics (CFRP) for the reason that the roll has high rigidity and light weight and allows the increase of operating speed of the printing press.
Since the roll which is made of a fiber-reinforced plastics has poor surface resistance to abrasion and possibly induces the phenomenon of scuffing, it has been proposed to plate its surface with a Cr layer for the purpose of liberating it from the drawbacks or to form on its surface a coat of a metallic or ceramic material by thermal spraying, a rather simple work, for the purpose of obviating the necessity of performing a troublesome plating work (JUM-B-04-7,378, JP-A-60-214,958, JP-A-61-96,063, and JP-A-61-104,061).
The guide roller which uses a roll of such fiber-reinforced plastics as mentioned above as its basis offer no perfect solution to its problem concerning the adhesion of ink or the defilement of the printed matter mentioned above when it has its surface coated with a layer of Cr formed by plating, a layer of a metallic material formed by thermal spraying, or a layer of a ceramic material formed by thermal spraying.
JP-A-04-310,741 and JP-A-06-207,614 have proposed rollers which are made of a fiber-reinforced plastics and which have the surfaces of the rollers coated with a fluorine resin, a silicone resin, or a fluorine and silicone-containing resin for the purpose of preventing the rollers from the defilement with ink.
The rollers which are protected solely by the coating with the resin have only sparing feasibility of being applied to such components as guide rollers which are used in the presence of an abrading action because their surface coating layers offer very poor resistance to abrasion and very quickly lose their sheet or web releasing effect owing to the abrasive contact.
Hereinafter, for the sake of simplifying this specification, regarding the printing sheet and web, the word "printing web" may be singly used in many parts of the specification. However, it would be easily understood for a person who is skilled in the art that the word may be read as the sense of the printing sheet as well as web.
In the light of such problems of the prior art as described above, the present inventors formerly proposed a printing web transporting roller which is obtained by superposing on a roller basis of fiber-reinforced plastics a composite coating film composed of a porous ceramic layer formed by thermal spraying and a layer of a resin having a low surface energy formed on the surface of the ceramic layer and inside the pores in the layer (JP-A-09-175,703).
In the printing web transporting roller, the ceramic layer formed by thermal spraying on the roller basis has a coarse surface combining short-cycle undulations (resembling pitch waves) with long-cycle undulations (resembling swell waves). When the ceramic layer is coated on the upper side thereof with the low surface energy resin by a procedure of impregnating the ceramic layer with this resin and drying the layer until the resin is solidified, the low surface energy resin layer is formed on the surface of the ceramic layer and inside the pores in the ceramic layer. The low surface energy resin, because the ceramic layer has undulations resembling pitch waves and acquires a porous texture as described above, seeps into the relevant depressions and manifests an anchoring effect consequently and forms a composite film with the ceramic layer owing to the satisfactory adhesiveness of the resin with the ceramic layer, with the result that the ceramic layer and the low surface energy resin layer will form the composite coating film. While the low surface energy resin covers the surface of the ceramic layer substantially wholly, it is deposited in a large thickness on the depressions shaped like pitch waves and in a small thickness on the protuberances shaped like pitch waves. The composite coating film, therefore, assumes a smooth surface as compared with the coating formed solely of the ceramic layer. The undulations originating in the ceramic layer formed by thermal spraying are not completely buried by the low surface energy resin and the undulations resembling swell waves are generally retained wholly. The coarse surface to be eventually formed, therefore, possesses smooth undulations.
When the printing web transporting roller contacts a printing web, therefore, the contact is not obtained on the entire roller surface but only on the surfaces of smooth protuberances. Since the surfaces engaging in this contact include the seats of the low surface energy resin, the transfer of the ink from the printing sheet does not occur easily and the ink which has been transferred at all can manifest an outstanding quality of being easily removed by a light contact of dry cloth because the surface is formed of the low surface energy resin and also because it has a profile of smooth undulations.
Further, since the layer of the low surface energy resin is deposited on the relevant surface as combined with the ceramic layer formed by thermal spraying as described above, it is not wholly abraded or peeled even after a very long use. It is worn only in the protuberances of the undulations resembling swell waves mentioned above, i.e. extremely minute sites. The roll surface, therefore, is allowed to maintain the low surface energy for a very long time and do not easily incur deterioration of properties. While the undulations resembling swell waves have been expressed as "swell" in comparison with the undulations of minute pitches, the swell is in such an extent as escapes visual detection completely. If the resin layer forming the surfaces of the protuberances is worn to expose the ceramic layer and the adhesion of ink and the reverse transfer of ink occur in the affected portions, the outcomes will not be so serious as to pose a problem from the standpoint of the quality of print.
When the printing web transporting roller constructed as described above is used for a long time, however, the layer of the low surface energy resin is worn and the ceramic layer is exposed on the minute protuberances. When the ceramic layer is exposed, though in very minute portions as described above, the adhesion of ink expands with the portions of exposure as cores possibly to the extent of entailing the problem of increasing the frequency of cleaning.
JP-B-07-119,103 and JP-B-07-119,104 disclose a guide roller which is provided on the roller surface with steps satisfying specific conditions and consequently allowed to allay the adhesion of ink by causing the roller surface to produce constant slippage with the printing web or sheets. Even the stepped guide roller has room for further improvement because the ink, once suffered to adhere to the roller surface, inevitably entails the problem concerning the removal of the adhering ink and the defilement of the printed matter with the persistent ink.